Metal containers for foods and beverages are generally made from aluminum, tin-free steel, blackplate and tinplate steel, (which is cold rolled steel sheet coated with a thin layer of tin). Because such metal containers are subject to air oxidation or rusting and to corrosion caused during the pasteurization and sterilization of the beer, soda pop, and various food products which are packaged in such containers, organic reactant polymer coating compositions are used to protect the metal from chemical attack and to protect the food products from the deleterious byproducts of chemical attack. Due to their potential contact with food products, such coatings present additional problems of toxicology and taste considerations.
Christenson et al. U.S. Pat. No. 4,097,438, as well as the patent references cited therein, refer to some of the problems of can coating for beverage and food product operations, and disclose water-based coating compositions for use as coatings for metal containers intended for beverage packages. In the Christenson '438 patent, the polymer is made in the absence of a mercaptan so that the polymer in the coating composition will not influence the taste of the beverage product in the polymer coated can. The Christenson U.S. Pat. No. 3,243,139 refers to can coating composition polymers made with mercaptans. Other of the patents listed in the Christenson '438 patent disclose coating composition polymers made in organic solvent media, but none of such patents recite all of the requirements of this invention.
Brown et al. U.S. Pat. No. 4,195,006 refers to Christenson et al. U.S. Pat. 3,991,216 which claims metal containers coated with the compositions claimed in Christenson et al. 4,065,415, among others, but distinguishes the Christenson '216 and '415 patent compositions in that the Brown et al. '006 patent adds 20 to 45 percent of acrylonitrile or methacrylonitrile monomer to its polymer ingredient of its coating composition, and is thus farther removed from the compositions of this invention.
McFadden U.S. Pat. No. 4,272,621 discloses water thinable coatings compositions of aminoalkylated polymers, polyepoxides and water miscible organic solvents made water soluble by acidification with hydrobromic acid or a mixture of hydrobromic acid and hydrochloric acid to provide improved polymer cure rates, but the McFadden '621 patent does not disclose the components of this invention.
An ideal aqueous coating composition, from the perspective of food and beverage manufacturing would meet the following criteria:
1) formulated from FDA-approved materials, PA0 2) fast curing at low temperatures, PA0 3) high solids, PA0 4) clean, color-free, shiny coating, PA0 5) cured coating should provide tack-free surface with frictional characteristics permitting easy movement in automated can-filling equipment, PA0 6) cured coating should be stable when subjected to high-temperature processing utilized to pasteurize or sterilize foods and beverages, PA0 7) cured coating should be stable when subjected to high temperatures in both acid and basic environments, PA0 8) coating should be capable of adhering to a wide variety of surfaces, including printed and decorated can surfaces, PA0 9) coating should exhibit good shelf life under a variety of atmospheric conditions. PA0 1. Check the can coating gloss and appearance vs. a standard (a previously approved standard batch of these compositions). The coated ca should have high gloss and clarity, good wetting and adhesion properties equal to the standard. PA0 2. MEK (methyl ethyl ketone) rubs: approximately 50+/-10. The actual number is recorded. This test estimates the can coatings degree of cure (reaction). PA0 3. Mobility: The Altek Coefficient of Friction (COF) test results are taken, PA0 4. 90 minutes@250.degree. F. Process Resistance test PA0 Test can coating against a standard. The coating on the test can should exhibit no delamination or loss of adhesion; can may exhibit slight blush equal to standard in both media.
Those in the art would prefer to have can coating polymer compositions that satisfy all of the required test parameters, instead of just some of them as well as permit faster can coating machinery speeds. Quality Control tests are made on can coated with these compositions and include:
a. &lt;0.07 after wash coat bake. PA1 b. &lt;0.07 after additional 3 min.@400.degree. F. PMT inside spray coating bake. PA1 c. &lt;0.08 after additional 10 min.@400.degree. F. TOT overbake. PA1 a. in Deionized Water PA1 b. in Deionized Water adjusted to pH 10.5 with sodium hydroxide (NaOH).
It is an object and purpose of this invention to provide thermoset organic polymeric metal can coating compositions of this nature that with only slight compositional variation:
(a) can be applied in accordance with known can coating methods such as by wash coat, spray coat, curtain coat, roll coat, or in Rutherford can coating machines at substantially faster ca coating line speeds than is possible with prior can coating compositions,
(b) will not produce an unacceptable color upon curing and will cure faster at lower bake temperatures, approximately 1 minute at 350.degree. F. PM to 15 seconds at 400.degree. F. PMT (PMT means peak metal temperature),
(c) will be compliant with EPA and FDA regulations in their manufacture and use in can coating operations, and in their association with beverage and food filling operations which follow, to obtain a can coating that upon curing will possess a sufficiently hard polymeric film surface to prevent can rub or contact marks and be sufficiently color-free to make for an acceptably appearing coated can while at the same time said coating will be open or permeable enough to permit the lubricant contained in the coating composition to bloom, migrate or move to the surface of the coating to facilitate a sufficient can mobility property between the coated cans so that the coated cans will move easily relative to each other and the canning equipment without any substantial rub, contact or scratch marks in can processing and beverage and food filling operations,
and which cured polymer coating will also have sufficient "process resistance" properties when exposed to the conventional pasteurization and sterilization temperatures and the varied pH conditions ranging from the acid side pH 5 to basic side pH 10.5 associated with waters typically encountered in beverage and food filling operations, without coating degradation to ensure an adequate can appearance and product shelf life.
It is also an object of this invention to provide beverage and food product metal cans and metal substrate therefore coated with compositions of this invention.